1. Field of the Invention
The present invention relates to a temperature control apparatus for controlling the temperature of a sensor element of a humidity sensor disposed in an exhaust passage of an internal combustion engine for detecting the humidity within the exhaust passage in order to maintain a good condition for the sensor element.
2. Description of the Prior Art
Some internal combustion engines are provided with an adsorbent in its exhaust system for adsorbing hydrocarbons in exhaust gases upon starting. The adsorbent has, for example, zeolite on the surface such that hydrocarbons in exhaust gases introduce into pores of zeolite when they pass through the adsorbent, and are adsorbed by the adsorbent. The adsorbent desorbs the hydrocarbons once adsorbed thereby when the adsorbent is heated by exhaust gases to a predetermined temperature or higher (for example, 100–250° C.), permitting the desorbed hydrocarbons to be recirculated to the internal combustion engine through an EGR pipe and the like. While the adsorbent repeats the adsorption and desorption of hydrocarbons as described above, the remaining amount of hydrocarbons not desorbed may gradually increase in the adsorbent, or pores of the adsorbent may be broken during a long-term use. As a result, the adsorbent is deteriorated, possibly causing a gradual degradation in the ability of the adsorbent to adsorb the hydrocarbons. It is therefore necessary to determine the state, more particularly, a deterioration of the adsorbent.
The applicant has already proposed a deterioration determining apparatus for determining a deterioration of an adsorbent as described above, for example, in Japanese Patent Application No. 2001-323811. This deterioration determining apparatus relies on a proportional relationship between the abilities of the adsorbent to adsorb hydrocarbons and moisture to determine a degradation in the abilities of the adsorbent to adsorb hydrocarbons and moisture, i.e., a deterioration of the adsorbent by detecting the humidity in exhaust gases using a humidity sensor after they pass the adsorbent. The humidity sensor comprises a sensor element which is made of a porous material having a large number of pores, and detects the humidity of exhaust gases as moisture in the exhaust gases introduces into the pores and is adsorbed therein when it passes through the sensor element. In this way, the humidity is detected with the sensor element exposed to exhaust gases, so that impurities such as water droplets produced by condensation, unburnt fuel components included in the exhaust gases, and the like can stick to the sensor element, in which case the humidity sensor fails to correctly detect the humidity of exhaust gases, resulting in the inability to correctly detect a deterioration of the adsorbent. To solve this problem, the deterioration determining apparatus performs heat cleaning by heating the sensor element using a heater for removing water droplets and the like sticking on the sensor element to recover the detection accuracy of the humidity sensor.
Specifically, in the heat cleaning, the heater is operated for the predetermined time period when the intake temperature is lower than a predetermined temperature before the start of the internal combustion engine, or when an idling operation continues for a predetermined time period or longer after the start of the internal combustion engine, on the assumption that the sensor element experiences condensation.
In the heat cleaning control described above, the heater is operated for a predetermined time period only in a situation in which it is assumed that the sensor element experiences condensation. However, during a normal operation of the internal combustion engine, impurities other than water droplets within exhaust gases may also stick to the sensor element. However, the heat cleaning is not performed when it is not assumed that the sensor element experiences condensation even if such impurities stick on the sensor element, resulting in a degraded accuracy of detection by the humidity sensor. In addition, since the heater is merely operated for a predetermined time period, the heat cleaning control may fail to sufficiently remove impurities or operate the heater for useless depending on, for example, a temperature state of an exhaust system and the like. Moreover, since the sensor element is suddenly heated by the heater in the presence of condensation, a sudden increase in the temperature of the sensor element from a low temperature, resulting from the heating, would possibly cause cracking of the sensor element.